configure.in: clock_gettime

* configure.in (clock_gettime): need to check with -lrt prior to check
  for the function only.  otherwise -lrt is not linked and the link
  fails, when ac_cv_func_clock_gettime is cached as yes.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42604 b2dd03c8-39d4-4d8f-98ff-823fe69b080e